In many applications, a position needs to be determined in two or more degrees of freedom. By way of example, a plurality of positions and/or situations of one-dimensional movement axes are usually determined in the case of coordinate measuring machines, which movement axes are linked to form coordinate information of a measuring head, by means of which measuring head geometric or other physical properties of the measurement object to be measured are determined. By way of example, another example are processing machines, in which a tool is intended to be measured or positioned in relation to a coordinate table. In addition to the conventional approach of using a Cartesian arrangement of two situation encoders, respectively for an X- and Y-direction, or for polar coordinates, to this end, it is a goal worth pursuing to detect the position using only a single situation encoder for both directions. Here, the area to which the established position and/or situation relates need not necessarily be planar but can also be embodied in a spherical fashion or as any free-form surface. Listed here as an example is a ball joint, movable in three degrees of freedom, in particular an articulation of an articulated arm or the like.
By way of example, for determining positions, EP 1 147 376 exhibits a phase measurement with an analog calculation of the correlation with sin/cos for incremental sensors. EP 1 750 099 complements such a phase measurement with the sin/cos correlation by an additional estimation of the frequency by means of an FFT (Fast Fourier Transform). For the correlation with sine and cosine functions, applied in the process, for detecting periodic patterns, use is generally made of area sensors such as CCD or CMOS sensors. With use of such array or area sensors, e.g. DE 699 33 050 describes a calculation of a correlation with sin/cos for incremental sensors. In EP 1 750 099, an estimation of the frequency with the aid of a Fast Fourier Transform (FFT) is also proposed in addition to the phase measurement. DE 60 2004 009 429 or EP 2 169 357 describe, inter alia, a summing of lines and/or columns of such an optical area sensor during the evaluation thereof.
Here, an incremental position determination, as finds use in the case of e.g. optical computer mice, is too imprecise or too susceptible to errors, particularly in the case of industrial applications. Therefore, an absolute determination of position in relation to an area should be provided, which renders it possible to determine a unique spatial position and, preferably, also a spatial situation of a sensor in relation to a reference surface or to an object to be measured. By way of example, EP 2 546 613 exhibits a nested encoding made of incremental and absolute code, or WO 2010/112082 shows a phase calculation of a Manchester-encoded absolute scale. EP 1 099 936 uses a combination of an absolute and relative 2D encoding. EP 1 473 549 shows an absolute encoding in each cell and EP 2 169 357 shows a nested encoding made of incremental and absolute code with an additional estimate of a twist by means of a 2D FFT.
Other examples of area codes are found, for example, in EP 2 133 824, U.S. Pat. No. 6,548,768, U.S. 2007/0246547 or WO 01/26032, with these not being continuous position codes for a position encoder for the precise determination of position and/or situation, which is applicable for industrial coordinate measuring machines, but area codes for characterizing relatively large regions with an absolute value assigned in each case to the region. By way of example, this is also connected to the fact that, in the documents above, the codes are only scanned, read and evaluated at points, whereas a code for a situation encoder sensor in accordance with the present invention moves continuously over the code area and, in the process, detects the position and/or orientation thereof with a resolution which goes beyond that of pure codeword information. Specifically, in an application according to the invention, it is therefore not only an encoding, but also a precise situation of the read code marks of the absolute encoding relative to the sensor which are evaluated.
Area codes according to the prior art are therefore usually incremental, or these locations do not constitute a continuous position code, but merely encode regions or areas with respectively associated codewords, such as e.g. in the case of code reader pens for paper wares in entertainment articles or the like. In these, the reader pen is always applied at a point and a codeword is established at this respective point. By contrast, embodiments of a present position code are embodied to such a purpose that a reading head is moved, preferably continuously, over the code area and position values are established in the process, said position values being more accurate than the pure, read absolute code values—i.e., in particular, a situation of one or more of the code value-forming code marks in relation to the reading-out sensor is also taken into account in the evaluation.
Moreover, in the prior art, a comparatively large code area portion usually needs to be detected by a correspondingly large sensor area in order to be able to encode a large area absolutely. Often, a considerable part of the code area is also used for a reference structure which does not contribute to the code value. Moreover, the coding laws of the codes used in the prior art are often complicated, leading to increased decoding outlay or slower evaluation.
In the field of the absolute area position encoding described here, a position and/or a twist should be determinable with high accuracy. In a specific embodiment, it is additionally also possible to determine tilts and/or a distance of the sensor from the encoded area, at least with a lower accuracy and/or a smaller measurement region than is the case in the primarily evaluated degrees of freedom of the 2D area code. As a result of this, it is possible, for example, to qualify and/or quantify possible maladjustments or deviations, e.g. as a result of adjustment errors, deformations, temperature expansions, etc., and these can be at least partly compensated mechanically or numerically.